Elements

Elements are sometimes called the building blocks of life. The periodic table (a copy is on the next

page) contains all the known elements and each one has a name and symbol.

Task 1

Use the Periodic Table to write down the symbols of the following elements.

a) Fluorine f) Palladium

b) Lithium g) Lead

c) Strontium h) Gold

d) Xenon i) Tungsten

e) Cobalt j) Mercury

Task 2

Use the Periodic Table to write down the names of the following elements.

a) Rn f) Ag

b) He g) Sn

c) Pt h) V

d) Zn i) H

e) Cs j) Rb

Challenge Task 1

What are the rules for writing the symbols of the elements?

Challenge Task 2

Research what the definition of an element is.

Challenge Task 3

Research who Dmitri Mendeleev is.

Challenge Task 3

Use the symbols of the elements to write down as many words as you can that contain seven or more

letters. You can use the symbols as many times as you want in each word.

The Periodic Table

Relative Atomic Mass and Atomic Number

Each element also has two numbers. The top number is called the relative atomic mass (or A r for short)

and the bottom number is called the atomic number. These are shown in the key on the previous page.

These numbers tell you how many protons, electrons and neutrons are found in each atom of each

element.

Number of protons = atomic number

Number of electrons = atomic number

Number of neutrons = relative atomic mass – atomic number

Task 1

Use this information (and the periodic table) to answer the following.

a) State the number of protons in an atom of magnesium.

b) State the number of protons in an atom of bromine.

c) State the number of electrons in an atom of carbon.

d) State the number of electrons in an atom of copper.

e) State the number of neutrons in an atom of iodine.

f) State the number of neutrons in an atom of rhenium.

Task 2

Name the element that doesn’t have any neutrons in its atoms.

Task 3

State how the elements are arranged in the Periodic Table.

Challenge Task 1

The charge of a proton is +1 and the charge of an electron is –1. Use this information to explain why all

atoms are neutral (have no charge) overall.

Challenge Task 2

The two atoms shown are the two isotopes of chlorine.

Use this information to state what the definition of an isotope is.

Chemical Formula

A chemical formula shows the number and type of each particle found in a substance.

Examples

a) Sodium chloride has the formula NaCl.

This tells us there is one sodium (Na) particle and one chlorine (Cl) particle.

b) Calcium bromide has the formula CaBr2.

This tells us there is one calcium (Ca) particle and two bromine (Br) particles.

c) Aluminium oxide has the formula Al2O3.

This tells us there are two aluminium (Al) particles and three oxygen (O) particles.

Task 1

Write the name and number of each particle in the following formulae.

a) NaF e) H2SO4

b) MgBr2 f) Rb2CO3

c) K2O g) CH3COOH

d) Fe2O3 h) C6H4N2C6H4NH2

Task 2

Nitric acid is an acid used to make fertilisers. Each molecule of nitric acid contains one atom of

hydrogen, one atom of nitrogen and three atoms of oxygen. Write the formula of nitric acid.

Task 3

Phosphoric acid is an acid that is found in some drinks. Each molecule of phosphoric acid contains

three atoms of hydrogen, one atom of phosphorus and four atoms of oxygen. Write the formula of

phosphoric acid.

Notice that if there is a number in a formula then it is written after the particle and below.

Examples

a) Magnesium hydroxide has the formula Mg(OH)2.

This tells us there is one magnesium (Mg) particle, two oxygen (O) particles and two

hydrogen (H) particles.

b) Calcium nitrate has the formula Ca(NO3)2.

This tells us there is one calcium (Ca) particle, two nitrogen (N) particles and six oxygen (O)

particles.

Task 4

Write the name and number of each particle in the following formulae.

a) Be(OH)2 e) Al2(CO3)3

b) Zn(NO3)2 f) Fe2(SO4)3

c) Fe(OH)3 g) CuSO4(H2O)3

d) (NH4)2SO4 h) C6H2(NO2)3CH3

Challenge Task 1

All the formulae in these tasks are of compounds. State what the definition of a compound is.

Challenge Task 2

Use the information in the table to match up each circle with each element then use your answers to

draw a diagram of CO2 and NH3.

If the formula contains a bracket with a number after it then this tells you to multiply everything

inside the bracket by the number.

Relative Formula Mass

A relative formula mass (or Mr for short) tells us the total mass of all the particles found in each

formula of a substance. To be able to calculate the M r we need to know the mass of the individual

particles, which is called the relative atomic mass (or Ar for short). The Ar of each atom can be found

using the periodic table. Each element has got two numbers and the Ar is the top number.

Example19 F

This shows that the Ar of fluorine (F) is 19

How to calculate the Mr of a substance

Work out how many particles there are of each element and then multiply by the Ar of that element.

Examples

a) Sodium chloride has the formula NaCl. This tells us there is one sodium (Na) particle and one

chlorine (Cl) particle.

23Na and 35.5Cl are the Ar values

Na x 1 = 23 x 1 = 23

Cl x 1 = 35.5 x 1 = 35.5

Mr of NaCl = 23 + 35.5 = 58.5

b) Calcium chloride has a formula CaCl2. This tells us there is one calcium (Ca) particle and two

chlorine (Cl) particles.

40Ca and 35.5Cl are the Ar values

Ca x 1 = 40 x 1 = 40

Cl x 2 = 35.5 x 2 = 71

Mr of CaCl2 = 40 + 71 = 111

c) Calcium nitrate has a formula Ca(NO3)2. This tells us there is one calcium (Ca) particle, two

nitrogen (N) particles and six oxygen (O) particles.

40Ca, 14N and 16O are the Ar values

Ca x 1 = 40 x 1 = 40

N x 2 = 14 x 2 = 28

O x 6 = 16 x 6 = 96

Mr of Ca(NO3)2 = 40 + 28 + 96 = 164

Task

Calculate the Mr of the following compounds.

a) NaF f) Ca(OH)2 k) Zn(NO3)2

b) MgBr2 g) Fe2(SO4)3 l) Al2(SO4)3

c) K2O h) Al2(CO3)3 m) (NH4)2CO3

d) Fe2O3 i) (NH4)2SO4 n) C6H3(NO2)3

e) H2SO4 j) Cu(NO3)2 o) C6H4(NH2)N2C6H3(OH)2

Challenge Task

In the following formulae there is a mystery particle, X. By finding the relative atomic mass of X,

identify the name and symbol of X.

a) XO (Mr XO = 25)

b) XO2 (Mr XO2 = 64)

c) X2O3 (Mr X2O3 = 160)

O = oxygen for each example

Homework 1

1. Write the name and number of each particle in the following formulae. (2 each)

a. KI b. SrO c. ZnCl2 d. CoCO3 e. (NH4)2SO4

2. Calculate the relative formula mass (Mr) of each of the following. (2 each)

a. CuSO4 b. Al2(CO3)3 c. Fe(OH)2

3. Calculate the relative atomic mass (Ar) of the unknown particle X in the following. (2 each)

a. XNO3 Mr = 170 b. X2O5 Mr = 182

Percentage (%) by Mass

To calculate the percentage (%) by mass of a certain particle (Z) in a substance you use the following

equation:

% by mass of Z = total mass of particles of Z x 100 Mr

Examples

a) Calculate the % by mass of hydrogen (H) in CH4. In this example, Z is hydrogen (H) and you need

to calculate the Mr of CH4.

Total mass of H (Z) Total mass of CH4 (Mr)

4 x 1 = 4 (1 x 12) + (4 x 1)

= 12 + 4

= 16

% by mass of hydrogen (H) = 4/16 x 100 = 25 %

b) Calculate the % by mass of oxygen (O) in CaCO3. In this example, Z is oxygen (O) and you need

to calculate the Mr of CaCO3.

Total mass of O (Z) Total mass of CaCO3 (Mr)

3 x 16 = 48 (1 x 40) + (1 x 12) + (3 x 16)

= 40 + 12 + 48

= 100

% by mass of oxygen (O) = 48/100 x 100 = 48 %

Remember that Mr is the relative formula mass.

Remember that you have already calculated some Mr values.

Task 1

Calculate the % by mass of hydrogen (H) in the following compounds. Give your answer to one decimal

place if it isn’t a whole number.

a) H2O

b) NaOH

c) C2H6

d) NaHCO3

e) H2CO3

f) (NH4)2CO3

Task 2

Calculate the % by mass of oxygen (O) in the following compounds. Give your answer to one decimal

place if it isn’t a whole number.

a) CO2

b) Li2O

c) KNO3

d) CH3COOH

e) H2SO4

f) (NH4)2SO4

Challenge Task 1

Calculate the percentage (%) by mass of water in CuSO4.5(H2O). Give your answer to one decimal place.

Challenge Task 2

Calculate the value of x in BaCl2.xH2O given that the Mr is 244.

Challenge Task 3

Calculate the value of x in Ca(NO3)2.xH2O given that the Mr is 236.

Ionic Formula

The formulae for some common ions are shown in the table.

Task 1

State what the definition of an ion is.

Task 2

What do you think “(II)” stands for after copper?

Task 3

State why iron is different to all the other names in the table.

Task 4

Why are the ammonium, carbonate, hydroxide, nitrate and sulfate ions different to all the others?

Task 5

The symbols of the five ions from task 4 now need to be written using a bracket. The letters are written

inside the bracket and everything else remains outside. Two have been done for you.

Examples

The ammonium ion, NH4+, is now written as (NH4)+

The carbonate ion, CO32–, is now written as (CO3)2–

How to write an ionic formula

1. Write the name of the two ions (one positive and one negative).

2. Write the symbols (and charges) of the two ions close to each other.

3. Rub out the + and – signs.

4. Swap over the numbers that are left then write them at the bottom.

5. If these numbers are the same then rub them out.

Examples

a) Sodium Iodide: Na+ I– = NaI

b) Potassium oxide: K+ O2– = K2O

c) Calcium oxide: Ca2+ O2– = CaO

d) Aluminium oxide: Al3+ O2– = Al2O3

e) Calcium nitrate: Ca2+ (NO3)– = Ca(NO3)2

Task 6

Write the chemical formula for each of the following compounds.

a) Sodium chloride d) Calcium fluoride g) Lithium Iodide

b) Magnesium bromide e) Copper(II) oxide h) Aluminium fluoride

c) Potassium oxide f) Silver bromide i) Iron(III) oxide

Task 7

Write the chemical formula for each of the following compounds.

a) Calcium hydroxide d) Iron(II) carbonate g) Potassium hydroxide

b) Sodium sulfate e) Ammonium hydroxide h) Aluminium nitrate

c) Ammonium chloride f) Zinc nitrate i) Iron(III) sulfate

Important rule – never change the inside of a bracket.

Challenge Task 1

The formula of lithium sulfide is Li2S. Write down the symbol and charge of the sulfide ion.

Challenge Task 2

Use your answer from challenge task 1 to write the formula of aluminium sulfide.

Challenge Task 3

The formula of sodium hydrogencarbonate can be written as NaHCO3. Write

down the symbol and charge of the hydrogencarbonate ion.

Challenge Task 4

Use your answer from challenge task 3 to write the formula of magnesium hydrogencarbonate.

Challenge Task 5

The formula of phosphoric acid can be written as H3PO4. Write down the symbol and charge of the

phosphate ion.

Challenge Task 6

Use your answer from challenge task 5 to write the formula of calcium phosphate.

Chemical Reactions and Mass

Reactants are turned into products in chemical reactions. The substances at the start are called the

reactants because they react together. The substances that are made are called the products because

they are produced.

reactants products

Mass is conserved in all chemical reactions. This means the total mass of the reactants will equal the

total mass of the products. This happens because no particles are destroyed and no new particles are

made.

Example

Sodium (2.3 g) reacts with fluorine (38.0 g) to produce sodium fluoride.

sodium + fluorine sodium fluoride

The mass of sodium fluoride produced = 2.3 g + 38.0 g = 40.3g

Task 1

For mass to be conserved in all chemical reactions, explain what has happened to the particles.

Task 2

The reactants, A and B, react together to produce a product, C.

A + B C

Use your knowledge of the conservation of mass to calculate the missing masses in the table.

Reaction Mass of A (g) Mass of B (g) Mass of C (g)

1 14 29

2 5.8 4.6

3 31 64

4 9.5 12.3

5 0.28 0.30

Task 3

The reactants, A and B, react together to produce the products, C and D.

A + B C + D

Use your knowledge of the conservation of mass to calculate the missing masses in the table.

Reaction Mass of A (g) Mass of B (g) Mass of C (g) Mass of D (g)

1 6 8 11

2 23 19 17

3 3.1 4.8 5.6

4 14.1 8.3 9.2

5 26.9 17.4 23.5

Task 4

Magnesium ribbon is heated in a Bunsen flame. The magnesium burns with a bright white light, as it

reacts with oxygen gas in the air, to form magnesium oxide. If the mass of magnesium used was 2.4 g,

explain whether the mass of the magnesium oxide formed be greater than, less than or equal to 2.4 g.

Challenge Task 1

The reactants, A and B, react together to produce the products, C, D and E.

A + B C + D + E

The mass of B that reacted was three times the mass of A and the mass of E produced was half of the

masses of C and D. If 7.5 g of A was used, calculate the masses of B, C, D and E.

Challenge Task 2

Write down some ways in which you would know a chemical reaction is taking place in a beaker.

Challenge Task 3

In an experiment, 100 g of a white solid, calcium carbonate (CaCO3) are strongly heated. At the end of

the experiment, the mass of the white solid remaining, calcium oxide (CaO), was 56 g. Give a reason

why the mass appears to have decreased in the reaction.

Percentage (%) Yield

The percentage (%) yield of a reaction tells us how much product has been produced. To calculate the

% yield you use the following equation:

% yield = mass of product obtained x 100 mass of product expected

Example

In a reaction, 25 g of product were expected but only 5 g were obtained.

% yield = 5/25 x 100 = 20 %

Task 1

State what the maximum % yield of a reaction can be.

Task 2

Calculate the % yields of the following reactions. Give your answer to one decimal place if it isn’t a

whole number.

Reaction Mass of product obtained (g) Mass of product expected (g)

1 3 12

2 15 25

3 4.6 7.9

4 15.4 19.7

5 67.8 84.3

Challenge Task

Give some reasons why the % yield of a reaction might be lower than expected.

The mass of product expected is sometimes called the

theoretical yield.

Homework 2

1. Calculate the percentage (%) by mass of carbon (C) in each of the following, giving your answer

to one decimal place. (2 each)

a. C3H8 b. CH3CO2C2H5

2. Write the ionic formula for each of the following substances. (1 each)

a. Calcium iodide b. Iron(III) nitrate c. Ammonium sulfate d. Magnesium oxide

3. In a chemical reaction mass is conserved. Explain what this means. (2)

4. 0.72 g of magnesium are completely reacted with oxygen gas to produce 1.20 g of magnesium

oxide as the only product. Calculate the mass of oxygen that reacted with the magnesium. (2)

5. 18.4 g of sodium react completely with 28.4 g of chlorine to produce sodium chloride as the

only product. The yield of sodium chloride produced was 50 %. Calculate the mass of sodium

chloride produced. (2)

Balancing Equations

A chemical reaction can be represented by a balanced equation.

Examples

Fe + S FeS MgCO3 Mg + CO2

2 Na + Cl2 2 NaCl 2 NaI + Br2 2 NaBr + I2

Task 1

By looking at the above examples, explain what is meant by a balanced equation.

Task 2

State which of the following equations are balanced.

a) Li2CO3 Li2O + CO2

b) 2 K + O2 2 K2O

c) Ca + 2 Cl2 CaCl2

d) Mg + H2SO4 MgSO4 + H2

e) 2 Na + 2 H2O 2 NaOH + H2

Task 3

The following equations all need to be balanced by placing numbers (except for the number 1) in the

gaps. Write out each equation with the correct number in each gap.

a) Ca + ___HCl CaCl2 + H2

b) ___Na + O2 ___Na2O

c) ___Li + Cl2 ___LiCl

d) ___H2O ___H2 + O2

e) ___Fe + ___Br2 ___FeBr3

f) ___Al2O3 ___Al + ___O2

There are numbers in front of some of the substances to balance the equation.

Challenge Task 1

The following equations all need to be balanced by placing numbers (except for the number 1) in the

gaps. Write out each equation with the correct number in each gap.

a) Ba3N2 + ___HF ___BaF2 + ___NH3

b) ___CaCl2 + ___Na3PO4 Ca3(PO4)2 + ___NaCl

c) ___Si2H2 + ___O2 ___SiO2 + ___H2O

Challenge Task 2

Magnesium hydroxide reacts with nitric acid (HNO3) to produce magnesium nitrate and water. Write

the balanced symbol equation for this reaction.

Moles and Standard Form

In chemistry, numbers of particles are counted in moles. A mole simply refers to a certain number of

particles. Other words are used to represent a certain number. For example:

1 dozen = 12 1 score = 20 1 gross = 144

This number is called the Avogadro constant. As the number shows, a mole is a huge number which is

why it is written in standard form:

Task

Write the answers to the following in standard form.

a) The number of seconds in an hour.

b) The number of seconds in a day.

c) The number of seconds in a year.

d) The number of seconds in 100 years.

e) The number of seconds in 1 million years.

f) The number of seconds in 100 million years.

1 mole = 6 x 1023

1 mole = 600,000,000,000,000,000,000,000

Calculating Moles of Atoms

To calculate the number of moles of individual atoms you use the following equation:

moles = mass (g) Ar

Example

Calculate the number of moles in 46 g of sodium.

mass of sodium = 46 g Ar of sodium = 23

moles = 46/23 = 2

Task

Calculate the number of moles of each the following atoms.

a) 28 g of lithium

b) 280 g of iron

c) 591 g of gold

d) 59.5 g of tin

e) 0.4 g of argon

Remember that Ar is the relative atomic mass.

Calculating Moles of Compounds

If you are not dealing with individual atoms then to calculate the number of moles you use the

following equation:

moles = mass (g)

Mr

This is the equation you use to calculate the number of moles of a compound.

Example

Calculate the number of moles in 54 g of water (H2O).

mass of water = 54 g Mr of water = 1 + 1 + 16 = 18

moles = 54/18 = 3

Task

Calculate the number of moles of each the following compounds.

a) 28 g of carbon monoxide (CO)

b) 192 g of sulfur dioxide (SO2)

c) 119 g of hydrogen peroxide (H2O2)

d) 16.2 g of hydrogen bromide (HBr)

e) 8.4 g of potassium hydroxide (KOH)

Challenge Task

Calculate the number of moles of the following substances. Give your answer to one decimal place if it

isn’t a whole number.

a) 1.0 kg of glucose (C6H12O6)

b) 0.3 kg of sulfuric acid (H2SO4)

c) 4 x 10-2 g of magnesium

Remember that Mr is the relative formula mass.

Calculating Masses using Moles

You can rearrange the equation on the previous page to calculate the mass of a substance.

For individual atoms For everything else, such as compounds

mass (g) = moles x Ar mass (g) = moles x Mr

Examples

a) Calculate the mass of 4 moles of sodium.

moles = 4 Ar of sodium = 23

mass = 4 x 23 = 92 g

b) Calculate the mass of 2 moles of water (H2O).

moles = 2 Mr of water = 1 + 1 + 16 = 18

mass = 2 x 18 = 36 g

Task

Calculate the mass (in g) of each the following substances.

a) 3.4 moles of helium

b) 2 moles of silver

c) 0.5 moles of rubidium

d) 1.5 moles of aluminium oxide (Al2O3)

e) 0.8 moles of nitric acid (HNO3)

Challenge Task 1

Identify each of the following elements using the information given.

a) 24 g and 0.5 moles

b) 7 g and 0.1 moles

c) 108 g and 9 moles

d) 9 g and 0.2 moles

e) 0.1 kg and 5 moles

Challenge Task 2

Calculate the number of oxygen atoms present in 2 moles of water (H2O).

Challenge Task 3

Calculate the number of hydrogen particles present in 0.5 moles of hydrogen bromide (HBr).

Hint

To answer challenge tasks 3 and 4 you need to use the Avogadro constant.

Simplest Formula

A simplest formula shows the smallest ratio of particles in a chemical formula.

Example 1

The chemical formula of ethane is C2H6

The simplest formula of ethane is CH3

The numbers in a simplest formula have to be whole numbers which means the chemical formula and

simplest formula might be the same.

Example 2

The chemical formula of carbon dioxide is CO2

The simplest formula of carbon dioxide is also CO2

Task 1

Write the simplest formula of each of the following compounds.

a) Water (H2O)

b) Butane (C4H10)

c) Pentane (C5H12)

d) Glucose (C6H12O6)

e) Ethanoic acid (CH3COOH)

To calculate a simplest formula using given masses you need to follow the following steps:

Step 1

Calculate the moles of each element.

Step 2

Divide each answer from step 1 by the smallest number.

Step 3

Write the simplest formula.

Remember that moles = mass Ar

Example 1

Calculate the simplest formula of a compound that contains 20 g of calcium and 80 g of bromine.

Step 1

moles of Ca = 20/40 = 0.5 moles of Br = 80/80 = 1

Step 2

The smallest number is 0.5 so divide the answers in step 1 by 0.5

Ca = 0.5/0.5 = 1 Br = 1/0.5 = 2

Step 3

The simplest formula is CaBr2

Example 2

Calculate the simplest formula of a compound that contains 24 g of magnesium and 16 g of oxygen.

Step 1

moles of Mg = 24/24 = 1 moles of O = 16/16 = 1

Step 2

The smallest number is 1 so divide the answers in step 1 by 1

Mg = 1/1 = 1 O = 1/1 = 1

Step 3

The simplest formula is MgO

Remember that moles = mass Ar

Remember that moles = mass Ar

Task 2

Calculate the simplest formula of each of the following.

a) A compound that contains 24 g of magnesium and 71 g of chlorine.

b) A compound that contains 23 g of sodium and 8 g of oxygen.

c) A compound that contains 15.6 g of potassium and 32 g of bromine.

Sometimes you have to calculate a mass first before you can calculate the simplest formula.

Example

An oxide of lithium, which had a mass of 45 g, contained 21 g of lithium.

The mass of oxygen in the oxide of lithium must be 45 g – 21 g = 24 g.

Now the steps are carried out:

Step 1

moles of Li = 21/7 = 3 moles of O = 24/16 = 1.5

Step 2

The smallest number is 1.5 so divide the answers in step 1 by 1

Li = 3/1.5 = 2 O = 1.5/1.5 = 1

Step 3

The simplest formula is Li2O

Task 3

An oxide of copper, which had a mass of 71.5 g, contained 63.5 g of copper. Calculate the mass of

oxygen in the oxide of copper and then calculate the simplest formula.

Task 4

A chloride of iron, which had a mass of 32.5 g, contained 11.2 g of iron. Calculate the mass of chlorine

in the chloride of iron and then calculate the simplest formula.

Challenge Task

Calculate the simplest formula of compound that contains 120 g of carbon and 25 g of hydrogen.

Homework 3

1. The following equations all need to be balanced by placing numbers (except for the number 1)

in the gaps. Write out each equation with the correct number in each gap. (1 each)

a. ___Rb + O2 ___Rb2O

b. ___Li + Br2 ___LiBr

2. State what number a mole is equal to. (1)

3. Calculate the number of moles in each of the following. (2 each)

a. 7 g of Si

b. 5 g of NaOH

c. 0.5 g of MgO

4. Calculate the mass (in g) in each of the following. (2 each)

a. 2 moles of Ne

b. 0.5 moles of CO2

c. 0.2 moles of NH3

5. Write the simplest formula of the following. (3)

a. C2H6

b. C3H6

c. C2H5

Molar Ratios

Chemical reactions can be represented by a balanced symbol equation. This shows the ratio of the

number of moles (the molar ratio) of reactants to products.

Example

Mg + 2 HCl MgCl2 + H2

This balanced symbol equation shows that 1 mole of Mg reacts with 2 moles of HCl to produce 1 mole

of MgCl2 and 1 mole of H2. Notice that if there is 1 mole of any substance then the number 1 doesn’t

need to be written in front of its symbol or formula in the balanced symbol equation.

The molar ratios of this reaction, which are the simplest possible, are shown below.

1 : 2 1 : 1

Task 1

Look at the following ratios and write the simplest ratio possible.

a) 1 : 3

b) 4 : 2

c) 12 : 3

d) 1.5 : 7.5

e) 0.375 : 0.125

Task 2

Use your knowledge of ratios to answer the following.

a) The ratio of apples to oranges is 1 : 1. State the number of oranges if there are 8 apples.

b) The ratio of boys to girls is 2 : 1. State the number of boys if there are 12 girls.

c) The molar ratio of sodium to lithium is 1 : 1. State the number of moles of sodium if there are

0.2 moles of lithium.

d) The molar ratio of water to carbon dioxide is 1 : 2. State the number of moles of water if there

are 0.5 moles of carbon dioxide.

e) The molar ratio of carbon to oxygen is 4 : 2. State the number of moles of carbon if there are

0.8 moles of oxygen.

Using Molar Ratios

You can be asked to calculate one of the following using molar ratios:

The mass of a product when the mass of a reactant is known

The mass of a reactant when the mass of a product is known

However, the same steps are followed for both:

Know Molar Ratio

(Know : Find)

Find

The moles of the “know” (reactant or product) will be calculated here

For individual atoms use:moles = mass/Ar

For everything else use:moles = mass/Mr

This is the molar ratio – you use the numbers in front of the “know” and “find” in the balanced symbol equation

This will allow you to calculate the moles (and then the mass) of the “find”

The mass of the “find” (reactant or product) will be calculated here

For individual atoms use:mass = moles x Ar

For everything else use:mass = moles x Mr

Example 1

Calculate the mass of MgCl2 that can be produced from 12 g of Mg.

Mg + 2 HCl MgCl2 + H2

Know Molar Ratio

(Know : Find)

Find

mass of Mg = 12 g Ar of Mg = 24

moles of Mg = mass/Ar

= 12/24 = 0.5

Mg : MgCl2

Remember that no number in front means 1 mole

1 : 1

0.5 : 0.5

This tells us the moles of MgCl2 are also 0.5

moles of MgCl2 = 0.5 Mr of MgCl2 = 95

mass of MgCl2

= moles x Mr

= 0.5 x 95 = 47.5 g

Example 2

Calculate the mass of Na that is needed to produce 0.62 g of Na2O.

4 Na + O2 2 Na2O

Know Molar Ratio

(Know : Find)

Find

mass of Na2O = 0.62 g Mr of Na2O = 62

moles of Na2O = mass/Mr

= 0.62/62 = 0.01

Na : Na2O

4 : 2

2 : 1

0.02 : 0.01

This tells us the moles of Na are 0.02

moles of Na = 0.02 Ar of Na = 23

mass of Na = moles x Ar

= 0.02 x 23 = 0.46 g

Example 3

Calculate the mass of H2 that can be produced from 4.6 g of Na.

2 Na + 2 H2O 2 NaOH + H2

Know Molar Ratio

(Know : Find)

Find

mass of Na = 4.6 g Ar of Na = 23

moles of Mg = mass/Ar

= 4.6/23 = 0.2

Na : H2

Remember that no number in front means 1 mole

2 : 1

0.2 : 0.1

This tells us the moles of H2

are 0.1

moles of H2 = 0.1 Mr of H2 = 2

mass of H2

= moles x Mr

= 0.1 x 2 = 0.2 g

Task 1

Use the “know / molar ratio / find” method to answer the following.

a) Calculate the mass of Mg that is needed to produce 0.8 g of MgO.

2 Mg + O2 2 MgO

b) Calculate the mass of CaO that can be produced from 10 g of CaCO3.

CaCO3 CaO + CO2

c) Calculate the mass of O2 that is needed to react with 3.9 g of K.

4 K + O2 2 K2O

Challenge Task

Calculate the mass of Al that is needed to react with 1.92 g of O2.

4 Al + 3 O2 2 Al2O3

Homework 4

1. A compound contained 6 g of magnesium and 4 g of oxygen. Calculate the simplest formula. (3)

Ar(Mg) = 24 Ar(O) = 16

2. 119 g of titanium chloride contained 48 g of titanium.

a. Calculate the mass of chlorine in the titanium chloride. (1)

b. Calculate the simplest formula for titanium chloride. (3)

Ar(Ti) = 48 Ar(Cl) = 35.5

3. An oxide of sodium, which had a mass of 6.2 g, contained 4.6 g of sodium.

a. Calculate the mass of oxygen in the oxide of sodium. (1)

b. Calculate the simplest formula for this oxide of sodium. (3)

Ar(Na) = 23 Ar(O) = 16

4. Calculate the mass of oxygen (in g) needed to react with 10 g of calcium. (3)

2 Ca + O2 2 CaO

Ar(Ca) = 40 Ar(O) = 16